1. Technical Field
The present invention relates to the field of multiple-color printing. More particularly, the present invention relates to the printing of multiple-color images onto a printing medium by the use of a field sequential printing technique, such as thermal transfer printing. Specifically, the present invention is directed to an assembly for circulating the paper onto which the multiple-color image is printed using a field sequential printing technique.
2. Background Information
Several methods for printing multiple-color images onto a printing medium are well known in the art. Some examples include ink-jet printing, electrophotography, electrostatic printing, and thermal transfer printing.
In ink-jet printing, the multiple color of the image are printed on the medium by multiple and individual minute nozzles which spray the medium with the appropriate color of ink. The minute nozzles are triggered according to image signals sent from a processor. This method, however, is not well suited for use in high speed printing operations. Additionally, due to the minute size of the nozzle outlet port and the inconsistency of the ink's density and flow characteristics, the device is inherently unreliable because the nozzles have a tendency to clog, thereby hindering ink flow and severely impending color image printing.
In electrophotograph and electrostatic printing, the color image is printed on the printing medium by transferring appropriately colored toner from a cylindrical drum to the printing medium. An electrostatic latent image is formed on the drum for each desired color, and the drum must be statically neutral prior to each application of a different toner color. In electrophotography, the latent image is formed by means of a laser; in electrostatic printing, the latent image is formed by means of a multi-stylus head. Both methods require as many developing units having toner containers as there are colors to be used. Accordingly, the printing apparatus is rather large and expensive.
The problems of reliability, speed, size and cost inherent with ink-jet, electrophotographic and electrostatic printing methods are generally solved by field sequential printing techniques such as thormal transfer printing devices. In thermal transfer printing, the color image is transferred to the printing medium by transferring colored ink from an ink ribbon to the medium by the application of heat developed at a thermal printing head. Accordingly, the printing process is simplified, with respect to the other types of printing techniques, because there is no need for separate developing units of toner as in the electrophotography or electrostatic printing method or separate ink nozzles as in ink-jet printing.
Turning now to FIG. 1, an example of the ink ribbon for use in thermal transfer printing is shown. As is commonly known to those of ordinary skill in the art, the ink ribbon comprises a base material to which thermally fusable solid ink is coated. The base material is usually either a paper or plastic derivative, and the colors of solid ink are typically yellow, magenta and cyanine. The ribbon's width corresponds generally to the largest width of paper and the horizontal length of the individual color fields of ink corresponds generally to the longest length of paper to be used. During the transfer process of the ink to the printing medium, the thermal printing head is triggered at appropriate points for each unique color of ink on the ribbon.
The ink on the ribbon can be considered to be separated into fields, and therefore thermal transfer printing is also referred to by those skilled in the art as field sequential printing. As used herein, the term "field sequential printing" refers to printing multiple-color images using a ribbon whose ink is segmented into fields such as the ribbon shown in FIG. 1, and should not be limited to the type of print head employed.
Turning now to FIG. 2, one arrangement of field sequential printing known in the art is shown. As shown in FIG. 2, paper 201 is conveyed in a forward direction by platen roller 202, driving roller 203 and pinch roller 204. Ink ribbon 205 is also conveyed in the forward direction at the same speed as the paper by suitable conveying means. The ink field of ink ribbon 205 comes in contact with paper 201 at the position where platen roller 202 is closest to the line of heat generating elements in thermal printing head 206. While the ribbon is conveyed in the forward direction, image signals are applied selectively to the heat generating elements, thereby printing the requisite portion of the image with the first color. Thereafter, platen roller 202 moves away from thermal printing head 206, and paper 201 is conveyed in a backward direction until the leading edge of the paper reaches the beginning print position. Then, ink ribbon 205 is conveyed until the beginning portion of the next color segment is reached. Thereafter, platen roller 202 returns to its print position and the printing of the next color is executed in the same manner as described above. The printing of the remaining colors are executed in the same fashion.
While the device shown in FIG. 2 reproduces color images using a field sequential printing technique, the dead time, caused by the necessary realignment of the paper back to its leading edge position for every color change, makes this type of system impractical for high speed printing applications. Other types of systems, such as that shown in FIG. 3, have been developed for trying to minimize or eliminate this dead time.
Turning now to FIG. 3, another arrangement of field sequential printing is shown. Recording medium 301 is would around drum-shaped platen 302 and comes in contact with the ink layer of ink ribbon 303 where the drum and printing head 304 are in closest contact. In this arrangement, the length of each segmented color layer corresponds to the circumference of platen 302. The printing of multiple colors is performed by continuously conveying ink ribbon 303 and rotating platen 302 at the same tangential velocity as that of the ink ribbon for the requisite number of cycles as determined by the number of unique colors on the ribbon. ehe thermal printing head is driven in accordance with the desired image to be printed. Paper 301 is separated from platen 302 after printing and discharged.
While the device shown in FIG. 3 minimizes dead time for paper lengths corresponding to the circumference of the platen, a substantial dead time will still be present when smaller-lengthed paper is used. Accordingly, the printing time corresponds to the size of the drum and is independent of paper size. Additionally, the device not only requires a complicated mechanism to wrap and separate the paper automatically, but the device also requires a special shaped thermal printing head. These requirements add significantly to the cost of the device. Furthermore, since the platen is large and requires a driving motor which will handle the inertia of the platen, it is not possible to package the arrangement in a small or compact way.